The hydrogen in the sub-surface ocean could combine with carbon dioxide molecules in a process known as "methanogenesis," which creates a byproduct of methane. If there are indeed microbes living in the moon's ocean, they could tap that energy source as sustenance.

Scientists said the moon appeared to have ample energy supplies to support life -- roughly the equivalent of 300 pizzas per hour, according to Christopher Glein, a geochemist at the Southwest Research Institute in Texas.

"This is the first time we've been able to make a calorie count of an alien ocean," he said.

'Pushing the frontiers'

Though Cassini does not have instruments capable of actually finding signs of life, "we've found that there's a food source there for it," said Waite.

"It would be like a candy store for microbes."

Jeffrey Seewald of the Woods Hole Oceanographic Institution echoed those comments in a companion article to the study: "This observation has fundamental implications for the possibility of life on Enceladus."

"Chemical disequilibrium that is known to support microbial life in Earth's deep oceans is also available to support life in the Enceladus ocean."

In a separate study published in The Astrophysical Journal Letters, scientists using the Hubble Space Telescope again found what is likely a plume emitting from Europa, one of Jupiter's four largest moons, which also has an icy crust atop an ocean.

After first spotting the apparent plume in 2014, scientists in 2016 saw it in the same spot, which appears to be a particularly warm region of Europa where fissures occur in the icy crust.

Both studies are laying the foundation for the Europa Clipper mission, which is slated to launch in the 2020s.

"If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them," said James Green, NASA's Planetary Science Division Director.

Cassini is slated to take a death plunge into Saturn's atmosphere in September, after it takes a final flyby of the giant moon Titan and a performs a series of 22 dives between the planet and its rings.

The decision to end the mission was made in 2010, in order to avoid damaging moons like Enceladus, which could be explored for signs of life in the future.

Researchers called its latest discovery a "capstone finding for the mission."

"We're pushing the frontiers. We're finding new environments," said Green.

"We're looking in a way that we never thought possible before for environments in our solar system which may harbor life today."